Transparent conductive films generally have high electrical conductivities and high transmittances in the visible light region. For these reasons, transparent conductive films are used as electrodes and the like for solar cells, liquid crystal display device, and other various light receiving devices, and also used as heat ray reflection films for automobile windows or building use, antistatic films, transparent heating members for preventing fog on freezer showcases or the like.
For the above-described applications, tin oxide containing antimony or fluorine as a dopant, zinc oxide containing aluminum or gallium as a dopant, indium oxide containing tin as a dopant, and the like have been used widely. In particular, films of indium oxide containing tin as a dopant, i.e., In2O3—SnO2-based films are called ITO (Indium tin oxide) films, and have been widely used so far, particularly because low-resistance films can easily be obtained.
In addition to electrical conductivity and transmittance, refractive index is also regarded as important, when a transparent conductive film is used as a part of a stacked film such as an optical film. Cerium oxide and titanium oxide have been generally used as high-refractive-index materials, but have poor electrical conductivities. Hence, when an electrical conductivity is required, the above-described cerium oxide or titanium oxide added to indium oxide or the like to provide an electrical conductivity is used.
Here, materials for vapor deposition for use in forming transparent conductive films by a vacuum vapor deposition method are broadly classified into two types; one is a material for vapor deposition made of particles each having a size of 5 mm less in diameter; the other is a tablet-shaped material having, for example, a diameter of about 30 mm and a height of about 10 mm.
Here, a vacuum vapor deposition method using the tablet-shaped material for vapor deposition (a tablet for vapor deposition) has the following problems. Specifically, when a tablet having a too-low density is irradiated with electron beams, the sintering of the tablet occurs rapidly, simultaneously with the evaporation of the material from the surface. As a result, the tablet partially shrinks, so that the tablet is broken. On the other hand, when a tablet having a too-high density is irradiated with electron beams, a difference in temperature is created between the surface and the inside of the tablet. As a result, breakage of the tablet (breakage due to thermal shock) occurs because of difference in thermal expansion. The occurrence of the breakage of the tablet makes it impossible to perform continuous film formation because of clogging of an apparatus with fragments, or results in deterioration in film qualities because nonuniform irradiation with the electron beams leads to variations in film formation conditions. Here, the deterioration in film qualities means deterioration in film thickness distribution and resistance distribution.
In this respect, in order to solve the above-described problems, a method is proposed for an ITO tablet for vapor deposition (also referred to as an ITO pellet for vapor deposition). In this method, an ITO sintered body having a relative density of 90% or more is crushed, and the obtained granules having grain diameters of 0.5 mm or less is sintered again, so that an ITO tablet having a relative density of 50% to 80%, both inclusive, is obtained (see Patent Document 1: Japanese Patent Application Publication No. Hei 11-100660).
Note that, the “relative density” refers to the ratio (%) of the density of a sintered body (a material for vapor deposition) to the calculated true density determined from true densities of mixed powders, which are starting materials of the material for vapor deposition, and is a value determined from the formula: (density of sintered body/calculated true density)×100=relative density of sintered body (%). In the case of ITO, the “calculated true density” is calculated as follows: calculated true density=100/{[ratio of indium oxide blended (% by mass)/true density of indium oxide]+[ratio of tin oxide blended (% by mass)/true density of tin oxide]}.
Meanwhile, for zinc oxide-based tablets doped with gallium or the like, a method is proposed in which the density of a sintered body is adjusted by using a powder calcined in advance as a part of a raw material powder (see Patent Document 2: Japanese Patent Application Publication No. 2006-117462).
Here, an attempt to produce a sintered body tablet of an indium oxide containing cerium as a dopant (hereinafter sometimes abbreviated as an ICO) by use of the method proposed in Patent Document 1 results in a problem that an ICO sintered body tablet having a relative density of 90% or more is difficult to crush because the ICO sintered body has a higher hardness than the above-described ITO sintered body. In addition, even when the ICO sintered body is crushed into a powder by some means, there is another problem that, in an attempt to mold the obtained powder into a tablet shape by pressing, the shape of the molded body cannot be retained, and the molded body is easily broken, because of poor collapsibility of the powder.
Meanwhile, with reference to the method proposed in Patent Document 2, it is made possible to produce an ICO sintered body tablet having a relative density of 50% to 80%, both inclusive, by using a calcined powder obtained by calcinating a powder mixture containing an indium oxide powder and a cerium oxide powder and also using a powder obtained by mixing an uncalcined indium oxide powder and an uncalcined cerium oxide powder. However, there is a problem that the tablet is cracked during electron beam deposition, even though the relative density is 50% to 80%, both inclusive.
The present invention has been made with attention focused on these problems. An object of the present invention is to provide a tablet for vapor deposition which is made of an indium oxide sintered body containing cerium as a dopant, and which is not broken even when irradiated with high-power electron beams, and to provide a method for producing the tablet for vapor deposition.